Forces,moments and stability

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Forces in equilibrium

Vectors and Scalars                                        pg90

Finding the resultant force of two forces acting on a object or each other in a given direction (angle)

Use pythag or trig to work out . 

A vector is any physical quantity that has a direction as well as a magnitude.

Scalar is any physical quantity that is not directional 

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Balanced forces

Equilibrium of a point object                                               pg94

When forces act on a point object, the object is in equilibrium (at rest of moving at a constant velocity) e.i balanced

support force S = Weight W 

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All moments have a turning effect and a centre of mass                             pg97

turning effect is the force at a pivot

centre of mass is the point at where an object is balanced (mid point of weight)

The moment of a force about any point is defined as the force x the perpendicular distance from the line of action of the force to the point 

principle of moents: sum of clockwise = sum of anticlockwise 

e.g for a sesaw to be balanced both side must have the same forces acting on them about the pivot point. 

moment of a force = f x d

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If a body in stable equilibrium is displaced the released,it returns to its equilibrium position. 

For example if a coat hanger from a support is displaced slightly, its swings bk to its equilibrium position

the centre of mass is always the equilibrium position of a object plus any force acting

tilting :

is where a force acts on a object which forces it to raise on one side 

force applied, centre of mass and height of object


finding the toppling point where the object has tilted so much it topples due to the force and centre of mass on a pivot point.

Slopes also act on this.                                                           pg100

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Equilibrium rules

For an object to be in Equilibrium all the forces acting on it must end up balanced           

weight W (centre of mass), support force S and friction F are the forces acting. 

f1 + f2 +f3 = 0 e.i it must balance out . 

normally calculated on slopes using free body diagrams which is a object with a centre of mass causing friction and a  support force perpendicular to the friction.               pg104

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static calculations

Pg 108 questions 

Static calculations finding the resultant forces on a object from a number of forces acting on it. 

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